1. Field of the Invention
The invention relates to polythiophene-based temperature indicators.
2. Description of Relevant Art
Polythiophenes are known for their electrically conductive properties. One technique used to study the electron flow is to analyze associated color changes when the temperature of the polythiophene is varied. Color changes provide insight into the electro-conductive properties of the polymer. There are numerous patent and literature citations which describe this work.
In many instances it is clearly desirable to know when an object or article reaches or has exceeded a specific temperature simply by viewing the object and noting that at least a portion of the object has exhibited a color change. Viewing includes visual observation by an individual or detection of color change by a sensor, which sensor would output a signal to be detected in any suitable manner.
As an example, in the food service industry there are hot trays and cold trays in which food is stored and/or served. If a cold tray, such as by regulation, is required to be maintained at a temperature of 38xc2x0 or lower then a sensor system might be in place to signal (alarm) that the temperature is above 38xc2x0 F. Alternatively, a thermometer might be used. However, in the food service industry, margins are thin, and unless required by regulation, sensing systems will not be used. There are enumerable situations where it would be desirable to provide a visual color indicator which would appear on an article if the temperature were unsafe, hot drink cups, stove tops, etcetera and/or not functioning properly, hot plates, freezers, etcetera, or if to know when desired temperatures were reached, e.g. ovens.
The present invention utilizes the color change characteristics of polythiophenes in a sensing system, which system will change color at a specific design temperature.
The polythiophene is generally of the structure: 
wherein R1-R6=a hydrogen, substituted or unsubstituted alkyl radical, substituted or unsubstituted alkoxy radical, substituted or unsubstituted aryl radical, substituted or unsubstituted thioalkyl radical, substituted or unsubstituted trialkylsilyl radical, substituted or unsubstituted acyl radical, substituted or unsubstituted ester radical, substituted or unsubstituted amine radical, substituted or unsubstituted amide radical, substituted or unsubstituted heteroaryl or substituted or unsubstituted aryl radical
n is between 1 and 1000,
m is between 0 and 1000, and
1 is between 1 and 1000.
The synthesized polythiophene is mixed with a carrier system or liquid medium. Depending upon the specific polythiophene used, the carrier system can be aqueous or organic. The polythiophene can be used in the carrier system as a mechanical separation, colloidial solution, or a molecular solution. Also, surfactants, anionic, cationic or non-anionic, can be used if necessary in the carrier system to ensure uniform distribution of the polythiophene in the system.
For the example described in the Background, a polythiophene is synthesized to exhibit the color change at about 38xc2x0 F. and maintain that color change at lower temperatures when used in a carrier system, which system is placed in heat transfer relationship with the tray (article) the temperature of which is being monitored. Conversely, if the hot tray is to be maintained at about 180xc2x0 F. or higher then a polythiophene is synthesized to change color at about 180xc2x0 F. The polythiophene is mixed in a carrier system, which carrier system is placed in heat exchange relationship, such as by coating, at least a portion of the tray.
In a preferred embodiment, polythiophene in an amount of 0.05 to 5.0% by weight based on the total weight of the system, preferably 0.2 to 0.8% weight, is mixed with an organic solvent. Suitable solvents include tetrahydrofuran, chloroform, methylene chloride, toluene, and N-methylpyrrolidone.
The system is generally applied to the article as a coating on an area of the article, or the entire article, which will be visible during the expected use of the article. The coating can be applied by any technique known in the art, such as by brush, roller, spraying, etc. Accordingly, the coatings typically have a thickness of 0.1 to 1000 microns. The carrier system can also be absorbed on a surface or both absorbed and adsorbed on a surface.
In another aspect of the invention, the system is comprised of polythiophenes that visually and reversibly change color at a prescribed temperature in the range of about xe2x88x9240-180xc2x0 C. and are thermally stable to high temperatures in a range of about 200-300xc2x0 C. The temperature of the color change of the polythiophenes, hereinafter the thermochromic transition, and the high and low temperature colors can be tailored by chemical modification of the polythiophenes.
In synthesizing a polythiophene for a specific design temperature, eg. for the series of poly(3-alkylthiophene)s there is roughly an inverse correlation with the length of the n-alkane substituent and the temperature of the thermochromic transition for both the regiorandom (R1=alkyl, R4=alkyl, n≅0.8, m≅0.2, 1=40xe2x88x9280, R2, R3, R5, R6=H) and regioregular (R1=alkyl, n=40xe2x88x9280, m=0, R2, R5, R6=H), poly(3-n-alkylthiophene)s. For regiorandom polymers longer substituents such as n-hexadecyl have lower temperature thermochromic transitions (81xc2x0 C.) than shorter chain substituents such as n-octyl (130xc2x0 C.). The regioregular polymers have higher thermochromic transitions than the regioregular polymers but the same inverse correlation with chainlength is observed. The n-hexadecyl and n-octyl have thermochromic transition centered around 125 and 175xc2x0 C. As long as the number of thiophene units in the polymer is approximately greater than sixteen the thermochromic transitions is molecular weight independent. Oligothiophenes (n+m+1 less than 16) have lower temperature thermochromic transitions than the polythiophenes (n+m+1 greater than 16).
Yet another aspect of the invention comprises paint, plastic or rubber composites comprised of the polythiophenes that are one color at temperatures below the thermochromic transition and are another color while above the transition. Both the low and high temperature colors and the temperature of the color change vary as a function of the substituent groups R1, R2, R3, R4, R5, and R6, the number of repeat units (1), and regioregularity of the repeat units (n and m).
The invention also comprises polythiophenes that can be used as pure compounds or can be incorporated into paints including polyurethanes, polysiloxanes, polyacrylates, and other related polymer-based paints and coatings with about 0.5% polymer based pigment with retention of the thermochromic behavior. The thermochromic polymer-based pigments can be incorporated via injection molding or extrusion into many commercially important plastics such as poly(ethylene terephthalate) (PET), polysytrene, polyethylene (HDPE and LDPE), other polyolefins, polydienes, polycarbonates, polyacrylics, polyacrylic acids, polyacrylamides, polymethacrylics, polyvinyl ethers, polyvinyl halides, poly(vinyl nitrile)s poly vinyl esters, polyesters, polysofones, polysulfonamides, polyamides, polyimines, polyimides, carbohydrates, and polymer mixtures and copolymers. The plastics retain a visually retrievable thermochromic response with pigment loadings of about 0.5% polymer-based pigment.
The sensor system can be used as a safety feature or a thermal sensor for stoves, baking utensils or pans, radiator caps, cooling racks, paper/plastic coffee cups and lids, baby bottles, cooking utensils, cooking ware, fire safety, food packaging, instrument sterilization, novelty items, food preparation and handling equipment, warning labels, packaging film, microwave dishes, frozen food packages, beverage bottles, cable or wire coverings, motor and engine parts, breaking systems, automobile or truck tires, bathtub coatings, and other substrates and/or articles where a visual indication of a temperature change is important.